FFmpeg: Same-Sized Input MP4s Concatenate into Differently Sized Outputs

[raywood]

I have a short MP4 and a long MP4. Both measure 1136 x 1704, according to MediaInfo. I want to concatenate these two MP4s losslessly. I listed them in cliplist.txt and then tried this:

ffmpeg -f concat -safe 0 -i cliplist.txt -c copy output.mp4

That concatenated correctly, but for some reason the first (i.e., short) MP4 was tiny (maybe 1/6 the dimensions of the long MP4) in output.mp4.

The short (introductory) MP4 began life as a PNG. I time-stretched it to last five seconds and used that to produce MP4 output using Adobe Premiere Elements. But I don't know why that would matter.

How can I get these two identically sized inputs to concatenate into a uniformly sized output.mp4?

[jagabo]

Your concat procedure appears to be correct. So you're saying the new video starts playing with a small frame size and later switches to a large frame size? Maybe your first mp4 was encoded with a small frame size and the header metadata was used to specify a larger playback?

[raywood]

Yes, that's the situation: starts small, then switches to large.

Would there be a way to eliminate Premiere Elements from the process, and just use FFmpeg to convert and concatenate that PNG to become a video prependix?

[jagabo]

The easiest approach is probably to re-encode the intro sequence with a large frame size, ie, upscale to the large frame size before encoding. Then you won't have this problem.

The other way is remux your existing file that plays oddly specifying the larger frame size at the container level. But that kind of flagging isn't obeyed by all players.

[raywood]

Not sure what else I can do in this regard. Both MP4s measure 1136 x 1704.

[jagabo]

Both MP4s measure 1136 x 1704.

I was speculating that the first video wasn't really 1136x1704 -- that it's really a smaller frame size but flagged for 1136x1704 playback. Then when you concat that information is lose, hence the smaller frame size on initial playback.

Can you make the videos available? Or maybe just the first?

[raywood]

I can't post the videos. But I can tell you what I did. I started with a PNG of the specified size, cropped from a larger PNG using IrfanView. I used Adobe Premiere Elements to convert it to a five-second MP4, specifying the dimensions I used in IrfanView.

They tell me APE can handle 4K. It did not balk at the specified 1136 x 1704 sizing.

I may have misstated the appearance of the output video. When I put PotPlayer into full screen view, it replays the input video as a smallish rectangle in a sea of black. So the output writing surface may be of the correct size, but for some reason the input video is not expanded to fill the available space as APE seemed to indicate it would be.

[jagabo]

Without seeing the actual videos all I can tell you is that I've used ffmpeg's concat filter many times and never had any problems like you are describing as long as each video has the same frame size. You can get unexpected behavior when using concat to join videos of different frame sizes -- different players will do different things when the frame size changes.

Note that most players have a setting that lets them keep the source frame size upon playback when playing full screen. Ie, a 640x480 video will play as a small picture with big black borders in the middle of a 1920x1080 display. Make sure you don't have that setting enabled.

[raywood]

I found a solution. Get away from Adobe Premiere Elements. Just use IrfanView's slideshow creator again. It produced the main body of the video; now (with different time duration setting) it produced the title screen and table of contents. Then the FFmpeg command worked just fine to concatenate. Not sure what the exact problem was, but this worked.

Thank you very much for your help. It really helped to have your confidence that this was supposed to work, that there ought to be a fix.